Thermal reflow of glass and fused silica body

ABSTRACT

Disclosed are synthetic silica glass body with a birefringence pattern having low fast axis direction randomness factor and glass reflow process. The glass reflow process comprises steps of: providing a glass tube having a notch; and thermally reflowing the glass tube to form a glass plate. The process can be advantageously used to produce fused silica glass plate without observable striae when viewed in the direction of optical axis. Also disclosed are optical members comprising the fused silica glass body and a process for reflowing glass cylinders.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/716,457 under 35 U.S.C. § 119, filed on Sep. 12,2005 and entitled “Thermal Reflow of Glass and Fused Silica Articles,”the content of which is relied upon and incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

The present invention relates to method of making glass articles bythermal reflow of glass and fused silica articles made by such method.In particular, the present invention relates to thermal reflow of glasstubes, as wall as silica glass bodies thus produced. The presentinvention is useful, for example, in producing high purity fused silicaplates from consolidated glass cylinder produced by using the OVD andIVD processes.

BACKGROUND OF THE INVENTION

Conventional methods for making plate glass include the float method,pressing and rolling. These methods have been used successfully inproducing soda-lime glasses and other glasses having a relatively lowsoftening temperature.

For glass materials with high softening temperatures, such as fusedsilica glass, these methods either cannot be used (such as the floatmethod using tin bath), or must be adapted to accommodate the highprocessing temperatures.

In many applications, high composition and property homogeneity of theglass used is required. For example, synthetic silica glass for use asoptical members in precision lithography tools operating in deep andvacuum UV regions, such as at about 248 nm or 193 nm, is required tohave a very high purity, very high compositional homogeneity andproperty homogeneity in terms of concentrations of metal ions anddistribution thereof, OH concentrations and distributions thereof,refractive index and variation thereof, transmission and variationthereof, laser damage resistance, birefringence and variation thereof,fictive temperature and variation thereof, and the like.

The conventional glass forming techniques mentioned above, such as theglass plate forming technologies, cannot be easily adapted for use inmaking high purity fused silica plate meeting the stringentcompositional and property requirements of the demanding applicationsmentioned above.

High purity synthetic silica glass are typically made via flamehydrolysis methods, such as outside vapor deposition (“OVD”), insidevapor deposition (“IVD”), vapor axial deposition (“VAD”),direct-to-glass methods, and the like. The glasses obtained directlyfrom these processes tend to have compositional and propertyinhomogeneity within the bulk. For example, striae caused bycompositional and/or refractive index variations may exist in glasscylinders obtained by OVD, IVD and VAD processes. Glasses made fromthese processes oftentimes need to be reshaped to a plate or otherconfiguration before further processing into optical elements. It ishighly desired that such striae be removed or minimized during suchreshaping for demanding applications, at least in the direction of theuse axis of the glass. However, conventional thermal reflow of the glasscylinder or pressing does not reduce the striae to a desired level formany demanding applications. Indeed, direct pressing of an OVD, IVD andVAD fused silica cylinder can lead to striae present and observable inthe direction of the optical axis of the glass.

As a result, various methods have been proposed in making fused silicaglass having a high level of homogeneity at least in the direction ofthe optical axis. These methods include reshaping processes and/orhomogenization processes. However, these currently existing approachesare limited in their ability to accomplish the task.

For example, Berkey and Moore (U.S. Pat. No. 6,689,516 B2) identified ameans to fabricate plates from OVD blanks, however the process islimited to thickness up to ˜16 mm. This process requires use ofelaborate fixturing to assist in reshaping (stretching) the glass withsecondary thermal processing to provide plate straightening. Otherapproaches employ the use of molds (U.S. Pat. No. 5,443,607); applyforce using various fixture designs (U.S. Pat. Nos. 4,358,306,5,443,607, United States Patent Application Publication Nos.20030115904A1 and 20030115905A1) to provide means for homogenizing thefused silica glass via reorienting, twisting or mixing striae and/orcompositional gradients. The use of these methods either are limited interms of the maximum mass, relative effectiveness for striae mixing, orare elaborate methods which may induce inclusions and other defects inthe glass due to extensive or multiple twisting and kneading operationson the glass surfaces to get the desired mixing action.

Moreover, it has been found that fused silica glass plates produced bythe prior art methods tend to have an undesired level of randomnessfactor in terms of the fast axis directions of the birefringence map.

Therefore, there remains the need for a process for reshaping and/orhomogenizing glass materials, particularly those having compositionaland/or property variations in the bulk, wherein the impact of suchcompositional/property variations are reduced or minimized. There isalso a need for high purity fused silica glass having a low level ofrandomness in terms of the fast axis directions in its birefringencemap.

The present invention satisfies these needs.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect of the present invention, it is provideda synthetic silica glass body having an optical axis and a birefringencepattern measured in a plane perpendicular to the optical axis in whichthe fast axis directions of the measured birefringence pixels have arandomness factor of between −0.50 and 0.50, preferably between −0.40and 0.40, more preferably between −0.30 and 0.30. In certainembodiments, the randomness factor is between −0.20 and 0.20.Preferably, when viewed in the direction of the optical axis of thesilica glass body, it is essentially free of striae. More preferably,when viewed in at least one direction perpendicular to the optical axisof the silica body, it is essentially free of striae. In one embodimentof the glass body of the present invention, it is a plate having twoessentially flat and essentially parallel major surfaces, each majorsurface having an area of at least 1 cm², preferably at least 4 cm²,more preferably at least 16 cm². In certain embodiments, each of themajor surfaces has an area of at least 100 cm². In other embodiments,each of the major surfaces has an area of at least 225 cm², such asabout 400 cm², 625 cm², 900 cm², or even larger.

Preferably, the synthetic silica glass body of the present invention hasa refractive index variation Δn as measured in a plane perpendicular tothe optical axis, wherein Δn≦10 ppm, preferably Δn≦5 ppm, morepreferably Δn≦2 ppm, most preferably Δn≦1 ppm.

Preferably, the synthetic silica glass body of the present invention hasan internal transmission at about 193 m of at least about 99.65% cm⁻²,more preferably at least 99.70% cm⁻¹, still more preferably at least99.75% cm⁻¹, still more preferably at least 99.80% cm⁻¹, most preferablyat least 99.85% cm⁻¹.

Preferably, the synthetic silica body of the present invention has a lowlevel of LIWFD.

Preferably, the synthetic silica body of the present invention has abirefringence of less than 5 nm/cm, preferably less than 3 nm/cm, morepreferably less than 1 nm/cm, most preferably less than 0.5 nm/cm, whenmeasured in a plane perpendicular to the optical axis.

Preferably, the synthetic silica body of the present invention has afictive temperature of lower than 1150° C., preferably lower than 1050°C., more preferably lower than 1000° C., most preferably lower thanabout 900° C.

A second aspect of the present invention is an optical element having anoptical axis which is made from the synthetic silica body is describedsummarily above and in greater detail below. Preferably, the opticalaxis of the optical element is parallel to the optical axis of thesynthetic silica body. In a preferred embodiment, the optical element isa lens element for use in lithographic device operating in deep orvacuum UV wavelength regions, such as about 248 nm, 193 nm and shorter.

A third aspect of the present invention is a process for making glassplate, comprising the following steps:

(I) providing a ready-to-flow notched glass tube having (a) alongitudinal center axis, and (b) an identified section between twocross-sections perpendicular to the tube center axis having alongitudinal section length L₁; and (c) a longitudinal notch in thedirection of the tube center axis of the ready-to-flow notched glasstube through the tube wall; and

(II) thermally reflowing the ready-to-flow notched glass tube at anelevated temperature to form a glass plate. Preferably, the notch of theready-to-flow notched glass tube has a center plane essentially parallelto the tube center axis of the ready-to-flow notched glass tube.Preferably, in step (II), the glass plate is formed to have two majorsurfaces and an optical axis essentially perpendicular to the two majorsurfaces. Preferably, in step (II) of the process of the presentinvention, the notched side and the notch of the glass tube face upwardsand the notched side is placed on the surface of a support.

The process of the present invention is particularly advantageous informing glass plates from glass tubes having striae, such as essentiallycircular striae when viewed in the direction of the rube center axis.

In a preferred embodiment of the process of the present invention, theglass tube is made of consolidated fused silica material.

In a preferred embodiment of the process of the present invention, theglass is high purity consolidated silica and step (II) is conducted inthe presence of a purifying atmosphere comprising a cleansing gas.Preferably, the cleansing gas comprised in the purifying atmosphere isselected from F₂, Cl₂, Br₂ and halogen-containing compounds, andcompatible mixtures thereof. The halogen-containing compounds may beselected from HF, HCl, HBr and compounds represented by the generalformula C_(a)S_(b)X_(c), where X is F, Cl, Br and combinations thereof,a, b and c are nor-negative integers meeting the valency requirements ofthe individual elements.

The present invention process is particularly advantageous for thermalreflow of consolidated silica glass cylinders made by using thesoot-to-glass processes, such as the OVD, IVD and VAD processes,especially those glass cylinders having circular striae when viewed inthe direction of its longitudinal axis.

In a preferred embodiment of the process of the present invention, instep (I), the notch is formed to have a center plane passing through thetube center axis of the ready-to-flow notched glass tube, and the twosides of the notch beside the center plane are essentially symmetricaround the center plane.

In a preferred embodiment of the process of the present invention, instep (I), the notch is formed to have an essentially rectangularcross-section when cut by a plane perpendicular to the tube center axisof the ready-to-flow notched glass tube.

In a preferred embodiment of the process of the present invention, instep (I), the notch is formed to have an essentially truncated “V” shapecross-section when cut by a plane perpendicular to the tube center axisof the ready-to-flow notched glass tube.

In a preferred embodiment of the process of the present invention, instep (I), the provided ready-to-flow notched glass tube has across-section that is part of a ring-shape defined by an essentiallycircular outer boundary having a diameter of OD₁ and an essentiallycircular inner boundary having a diameter of ID₁ when cut by a planeperpendicular of the tube center axis of the tube. Preferably, the outercircular boundary and the inner circular boundary are concentric. In oneembodiment, however, the outer circular boundary and the inner circularboundary are eccentric. In this latter embodiment, it is preferred thatin step (I), the notch is formed at the location such that the centerplane of the notch is located where the thickness of the wall of theready-to-flow notched glass tube is essentially the minimal.

In yet another preferred embodiment of the process of the presentinvention, in step (II), the identified section of the ready-to-flownotched glass tube is formed into a glass plate having two essentiallyflat major surfaces, a width of a first major flat surface of L₃, awidth of a second major surface of L₄, L₄≧L₃, a length of both majorsurfaces of L₂, and a thickness between the two essentially flat majorsurfaces of T. Preferably, L₁≦L₂≦2L₁, more preferably L₁≦L₂≦1.5L₁, stillmore preferably L₁≦L₂≦1.2L₁. In a preferred embodiment of the process ofthe present invention, L₄≧L₃≧0.8 L₄, preferably L₄≧L₃≧0.9L₄, morepreferably L₄≧L₃≧0.95L₄.

In a preferred embodiment of the process of the present invention, instep (II), the identified section of the ready-to-flow notched glasstube is formed into a glass plate having two essentially flat majorsurfaces, a width of a first major flat surface of L₃, a width of asecond major surface of L₄, L₄≧L₃, a length of both major surfaces ofL₂, and a thickness between the two essentially flat major surfaces ofT. It is preferred that 0.5(π·OD₁−L_(arc))≦L₄≦2(π·OD₁−L_(arc)),preferably 0.5(π·OD₁−L_(arc))≦L₄≦1.8(π·OD₁−L_(arc)), more preferably0.7(π·OD₁−L_(arc))≦L₄≦1.5(π·OD₁−L_(arc)), where L_(arc) is the outer arclength of the notch. It is also preferred that L₃≧1.0·π·ID₁, morepreferably L₃≧1.5π·ID₁, still more preferably L₃≧2π·ID₁, still morepreferably L₃≧3π·ID₁. Meanwhile, it is also preferred that0.10·(OD₁−ID₁)≦T≦0.45·(OD₁·ID₁), preferably0.10·(OD₁−ID₁)≦T≦0.40·(OD₁−ID₁), more preferably0.10·(OD₁−ID₁)≦T≦0.30·(OD₁−ID₁).

Preferably, in the process of the present invention:

in step (II), the identified section of the ready-to-flow notched glasstube forms an identified glass plate having two essentially flat majorsurfaces, a width of the first major flat surface of L₃, a width of asecond major surface of L₄, L₄≧L₃, a length of both major surfaces ofL₂, and a thickness between the two essentially flat major surfaces ofT; and

measured in a plane perpendicular to the optical axis of the identifiedglass plate, the identified glass plate upon edge removal and surfacelapping with a surface area of about L₃·L₂ has a birefringence patternin which the fast axis directions have a randomness factor of between−0.50 and 0.50, preferably between −0.40 and 0.40, more preferablybetween −0.30 and 0.30.

In one embodiment of the process of the present invention, step (I)comprises the following steps:

(Ia) providing a precursor glass tube having (a) a longitudinal tubecenter axis, and (b) an identified section between two cross-sectionsperpendicular to the tube center axis having a longitudinal sectionlength L₁; and

(Ib) forming a notch in a direction parallel to the tube center axis ofthe precursor glass tube through the tube wall, whereby theready-to-flow notched-glass tube is formed.

In a preferred embodiment, the glass is silica and step (Ia) comprisesthe following steps:

(Ia1) forming a silica soot preform by the OVD process on a mandrel;

(Ia2) consolidating the silica soot preform into fused silica glasswithout previously removing the mandrel; and

(Ia3) removing the mandrel to form the precursor glass tube.

In another preferred embodiment, the glass is silica and step (Ia)comprises the following steps:

(Ia1) forming a silica soot preform by the OVD process on a mandrel;

(Ia2) removing the mandrel from the soot preform; and

(Ia3) consolidating the silica soot preform into fused silica glass,whereby the precursor glass tube is formed.

In yet another preferred embodiment, the glass is silica and step (Ia)comprises the following steps:

(Ia1) forming a silica soot preform by the OVD process on a glass tubemandrel; and

(Ia2) consolidating the silica soot preform into fused silica glasswithout previously removing the mandrel, whereby the precursor glasstube is formed.

In this preferred embodiment, the process comprises the following step(III) after step (II):

(III) removing the surface part of the glass plate resulting from theglass tube mandrel.

According to another preferred embodiment, the glass is silica and step(Ia) comprises the following steps:

(Ia1) forming a silica soot preform by the IVD process on the innersurface of an outside tube;

(Ia2) consolidating the silica soot preform into fused silica glasswithout previously removing the outside tube; and

(Ia3) removing the outside tube to form the precursor glass tube.

According to another preferred embodiment, the glass is silica and step(Ia) comprises the following steps:

(Ia1) forming a silica soot preform by the IVD process on the innersurface of an outside tube;

(Ia2) removing the outside tube from the soot preform; and

(Ia3) consolidating the silica soot preform into fused silica glass,whereby the precursor glass tube is formed.

According to yet another preferred embodiment, the glass is silica andstep (Ia) comprises the following steps:

(Ia1) forming a silica soot preform by the IVD process on the innersurface of an outside tube; and

(Ia2) consolidating the silica soot preform into fused silica glasswithout previously removing the outside tube, whereby the precursorglass tube is formed.

In this preferred embodiment, it is further preferred that it comprisesthe following step (III) after step (II):

(III) removing the surface part of the glass plate resulting from theoutside tube.

It is further preferred that step (Ia) comprises the following steps:

(I0) providing a precursor glass cylinder having a precursor cylinderaxis, a length L₀ in the direction of the precursor cylinder axis and aprecursor cylinder outer diameter OD₀;

(I1) thermally reflowing in the longitudinal direction of the precursorglass cylinder, with optional pressing; and

(I2) optionally drilling in a direction essentially parallel to theprecursor cylinder axis to form a cylindrical inner cavity,

whereby the precursor glass tube is formed to have a longitudinal tubeaxis, an outer diameter OD₁ and a length L₁ in the direction of the tubeaxis, where the tube axis is essentially parallel to the precursorcylinder axis of the precursor glass cylinder, L₁<L₀, and OD₁>OD₀.Preferably, the tube axis is the same as the precursor cylinder axis ofthe precursor glass cylinder.

In the preferred embodiment described above, 0.3L₀≦L₁≦0.8L₀.

In the preferred embodiment described above, it is preferred that:

in step (I0), the precursor glass cylinder comprises an inner glasscane; said inner glass cane is located approximately at the center ofthe precursor glass cylinder and has a diameter of ID₀; The glass canemay optionally have the same or a differing composition and/orproperties than the glass surrounding the inner glass cane; and

in step (I2), the inner glass cane is essentially completely removed.

In an embodiment of the process of the present invention, in step (I0),the precursor glass cylinder comprises a mandrel in essentially thecentral portion. The mandrel may be maintained in place during step(I2), and removed after step (I2). In one embodiment, the dimension ofthe mandrel is essentially not changed during step (I2). In oneembodiment, the mandrel is inserted into a glass tube. In anotherembodiment, in step (I0), the precursor glass cylinder comprises anoutside tube having differing composition and/or properties. In thisembodiment, it is further preferred that the process comprises thefollowing step (III) after step (II):

(III) removing the surface part of the glass plate resulting from theoutside tube.

In this preferred embodiment of the process of the present invention, itis further preferred that after step (I2), the ready-to-flow notchedglass tube has an inner cylindrical cavity with a diameter ID₁, andOD₀−ID₀<OD₁−ID₁.

In an embodiment of the process of the present invention, in step (I0),the provided precursor glass cylinder has an inner cylindrical cavitythe axis of which is parallel to the precursor cylinder axis, and theinner cylindrical cavity has a diameter of ID₀. Preferably, an innercylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁. A mandrelmay be inserted into the inner cylindrical cavity of the precursor glasscylinder during steps (I) and/or (II).

In one embodiment of the process of the present invention, in step (II),the reflow is done without external mechanical assistance. The externalforce exerted on the glass tube during reflow other than those by thevessel in which the reflow is carried out may include only gravity ofthe glass tube, or additional force.

In one embodiment of the process of the present invention, externalforces other than gravity of the glass tube are exerted on theready-to-flow notched glass tube to facilitate the reflow of the glass.Such external forces other than gravity of the glass tube may be exertedto the two side surfaces of the ready-to-flow notched glass tube and/orto the surfaces of the inner cavity thereof. In a preferred embodiment,the external force is applied by a plunger to the surfaces of the innercavity and/or the side surfaces of the notch. In another preferredembodiment, the external force is applied via an articulating mandreland a plunger.

In another embodiment of the process of the present invention, step (II)comprises the following steps:

(IIa) placing the ready-to-flow notched glass-tube-on an essentiallyhorizontal longitudinal mandrel, with the mandrel inserting into theinner cavity of the tube, and the notch placed facing sideways;

(IIb) allowing the lower part of the notched glass tube to roll out toan essentially vertical position while restricting the upper part fromrolling out, to result in a partially rolled out glass piece;

(IIc) placing the partially rolled out glass piece on a surface; and

(IId) allowing the partially rolled out glass piece to roll-out on thesurface to form an essentially flat glass plate. Preferably, in step(IId), an external force is imposed on the partially rolled-out glasspiece to mechanically assist the roll-out of the glass piece.Preferably, the external force is imposed via a mandrel.

According to one embodiment of the process of the present invention, theglass tube has essentially circular striae when viewed in the directionof the tube center axis, and after step (II), the striae are re-orientedto be essentially parallel to the two major surfaces of the resultantglass plate.

According to one embodiment of the process of the present invention, theglass tube has essentially circular striae when viewed in the directionof the tube center axis, and after step (II), when viewed in thedirection of the optical axis of the resultant glass plate, the glassplate is essentially free of striae. In addition, in certainembodiments, when viewed in at least one direction perpendicular to theoptical axis of the resultant glass plate, such as in the direction ofthe center tube axis of the ready-to-flow notched glass tube, the glassplate is essentially free of striae.

In a preferred embodiment of the process of the present invention, instep (II), the temperature elevation rate is between 50-600° C./minute,preferably between 180-600° C./minute between the annealing point of theglass and the highest temperature. Preferably, in step (II), thetemperature is held for a period of between 10 minutes to 5 hours,preferably between 10 minutes and 3 hours, at a temperature between theannealing point and the devitrification range of the glass.

A second aspect of the present invention is a process for reformingglass cylinders, comprising the following steps:

(I.0) providing a precursor glass cylinder having a precursor cylinderaxis, a length L₀ in the direction of the precursor cylinder axis and aprecursor cylinder outer diameter OD₀;

(I.1) thermally reflowing, with optional pressing, the precursor glasscylinder; and

(I.2) optionally drilling in a direction essentially parallel to theprecursor cylinder axis to form a cylindrical center cavity,

whereby a reformed glass cylinder is formed to have a longitudinalreformed cylinder axis, an outer diameter OD₁ and a length L₁ in thedirection of the reformed cylinder axis, where the reformed cylinderaxis is essentially parallel to the precursor glass cylinder axis of theprecursor glass cylinder, L₁<L₀, and OD₁>OD₀.

Preferably, in the glass cylinder reforming process of the presentinvention, 0.3L₀≦L₁≦0.8L₀.

Preferably, in the glass cylinder reforming process of the presentinvention:

in step (I.0), the precursor glass cylinder comprises an inner glasscane; said inner glass cane is located approximately at the center ofthe precursor glass cylinder and has a diameter of ID₀; and

in step (I.2), the inner glass cane is essentially completely removed.

Preferably, in the glass cylinder reforming process of the presentinvention, in step (I.0), the precursor glass cylinder comprises amandrel in essentially the central portion. Preferably, the mandrel ismaintained in place during step (I.1), and removed after step (I.1).Preferably, the dimension of the mandrel is essentially not changedduring step (I.1). The mandrel may be inserted into a glass tube.

Preferably, in the glass cylinder reforming process of the presentinvention, in step (I.0), the precursor glass cylinder comprises anoutside tube having composition and/or properties similar to ordiffering from those of the glass enclosed in the outside tube.Preferably, this preferred embodiment further comprises the followingstep (III) after step (II):

(III) removing the surface part of the glass plate resulting from theoutside tube. Preferably, in the glass cylinder reforming process of thepresent invention, after step (I.2), the precursor glass tube has aninner cylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁.

Preferably, in the glass cylinder reforming process of the presentinvention, in step (I.0), the provided precursor glass cylinder has aninner cylindrical cavity the axis of which is parallel to the precursorglass cylinder axis, and the inner cylindrical cavity has a diameter ofID₀. Preferably, the ready-to-flow glass tube has an inner cylindricalcavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁.

The present invention process can be applied to thin and thick wallcross-section cylindrical blanks to attain thick photolithography lensblanks. No elaborate fixturing or apparatus is required for glassmanipulation in the reshaping process of the present invention.Additionally, no secondary thermal treatments are required for platestraightening. This invention also allows for reorientation ofconcentric striae and radial compositional gradients, as seen in OVDcylindrical blanks, to favorable orientation to attain the requiredoptical properties.

Surprisingly, the high purity fused silica glass of the presentinvention, which can be produced by using the reshaping method of thepresent invention, has a low level of fast axis direction randomnessfactor in its birefringence pattern.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that the foregoing general description and thefollowing detailed description are merely exemplary of the invention,and are intended to provide an overview or framework to understandingthe nature and character of the invention as it is claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a schematic drawing illustrating steps (I0), (I1) and (I2) ofcertain embodiments of the process of the present invention, wherein aprecursor glass tube is formed.

FIG. 2 is a schematic drawing illustrating step (Ib) of certainembodiments of the process of the present invention, wherein a notch isformed in the precursor glass tube to produce the ready-to-flow notchedglass tube.

FIG. 3 is a schematic drawing illustrating step (II) of certainembodiments of the process of the present invention, wherein theidentified section of the ready-to-flow notched glass tube is reflowedto form a glass plate having two essentially flat major surfaces, awidth of a first major surface of L₃, a width of a second major surfaceof L₄, and a length of both major surfaces of L₂.

FIG. 4 is a schematic drawing illustrating the cross-section of anembodiment of a precursor glass tube cut by a plane perpendicular to thelongitudinal center tube axis before notch formation having circularstriae in the cross-section.

FIG. 5 is a schematic drawing illustrating the cross-section of theembodiment of a ready-to-flow notched glass tube corresponding to theprecursor glass tube of FIG. 4 after notch-formation, wherein the notchhas essentially a rectangular cross-section when cut by a planeperpendicular to the longitudinal center axis of the glass tube, and therectangular cross-section has a width less than the inner diameter ofthe glass tube.

FIG. 6 is a schematic drawing illustrating the cross-section of theembodiment of a ready-to-flow notched glass tube corresponding to theprecursor glass tube of FIG. 4 after notch-formation, wherein the notchhas essentially a rectangular cross-section when cut by a planeperpendicular to the longitudinal center axis of the glass tube, and therectangular cross-section has a width substantially equal to the innerdiameter of the glass tube.

FIG. 7 is a schematic drawing illustrating the cross-section of theembodiment of a ready-to-flow notched glass tube corresponding to theprecursor glass tube of FIG. 4 after notch-formation, wherein the notchhas essentially a truncated V-shaped (trapezoidal) cross-section whencut by a plane perpendicular to the longitudinal center axis of theglass tube, and the shorter base line of the trapezoidal cross-sectionhas a width less than the inner diameter of the glass tube.

FIG. 8 is a schematic drawing illustrating the cross-section of theembodiment of a ready-to-flow notched glass tube corresponding to theprecursor glass tube of FIG. 4 after notch-formation, wherein the notchhas essentially a trapezoidal cross-section when cut by a planeperpendicular to the longitudinal center axis of the glass tube similarto that of FIG. 7, but the shorter base line of the trapezoidalcross-section in this figure is longer than that in FIG. 7.

FIG. 9 is a schematic drawing illustrating the cross-section of a glassbody extracted from the glass plate showed in FIG. 3, cut by a planeperpendicular to the center axis of the ready-to-flow notched glasstube. The glass body has a plurality of striae essentially parallel toeach other in the cross-section.

FIG. 10 is a schematic drawing illustrating a device in which a glasscylinder is longitudinally reflowed under weight to a cylinder havingshorter length at an elevated temperature.

FIG. 11 is a schematic drawing illustrating the notched glass tube ofFIG. 7 being reflowed with the mechanical assistance of external forcesexerted via stretching arms to the side surfaces of the notch.

FIG. 12 is a schematic drawing illustrating a step of the roll-out ofthe notched glass tube of FIG. 5, in which half is reflowed undergravity, and the other half is restricted from reflow by mechanicalassistance of a mandrel and an upper fixture.

FIG. 13 is a schematic drawing illustrating the roll-out of the halfreflowed glass piece showed in FIG. 12 on a slope.

FIG. 14 is a schematic drawing illustrating the roll-out of a notchedtube approximating the configuration in FIG. 7 via the mechanicalassistance of a hinged articulating mandrel and a plunger.

FIG. 15 is a birefringence map (showing directions of the fast axesonly) of a piece of fused silica glass having a tangential pattern offast axis direction distribution.

FIG. 16 is a birefringence map (showing directions of the fast axesonly) of a piece of fused silica glass having a radial pattern of fastaxis direction distribution.

FIG. 17 is a birefringence map (showing directions of the fast axesonly) of a piece of fused silica glass of the present invention having amixed pattern of fast axis direction distribution.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “variation of refractive index,” or “refractiveindex variation,” or “Δn,” means the maximal variation of refractiveindices measured in a plane perpendicular to the optical axis of theglass body or glass optical member along a predetermined direction byusing interferometry at about 633 nm (He—Ne laser) (with tilt and pistontaken out, as indicated infra). As is typically done by one skilled inthe art, when discussing refractive index variation along a certaindirection, tilt and piston are subtracted. Therefore, the refractiveindex variation along a certain direction (such as the direction of thex axis or they axis of the three-dimension orthogonal coordinate systemillustrated in FIG. 3) in the meaning of the present application doesnot include tilt or piston. As indicated below, typically, the opticalaxis of a glass optical member, a glass blank, or a piece of glassmaterial, is selected to be perpendicular to a plane (a cross-section)in which the measured refractive index inhomogeneity is the smallest, inorder to obtain a glass member having large clear aperture area. FIG. 3in the drawings of the present application schematically illustrates aglass body of the present invention in a xyz orthogonal coordinatesystem. The glass body has an optical axis z. The plane xOy,perpendicular to axis z, intersects the glass body to obtain across-section of the blank. When measuring refractive index homogeneity,the sample taken has a uniform thickness. Preferably, when measuredacross the cross-section, the variation of refractive index of the glassbody of the present invention in the desired direction (such as the xdirection as illustrated in FIG. 3), with tilt and piston taken out, isless than 10 ppm, preferably less than 5 ppm, more preferably less than2 ppm, still more preferably less than 1 ppm, most preferably less than0.5 ppm. Desirably, the variation of refractive index in both the x andy direction, measured separately, with tilt and piston taken out, isless than 10 ppm, preferably less than 5 ppm, more preferably less than2 ppm, still more preferably less than 1 ppm, most preferably less than0.5 ppm.

The birefringence of the glass is measured by a polarimeter at 633 nm(He—Ne laser) in accordance with methods well established in the art,using, for example, commercially available instruments specificallydesigned for measuring birefringence.

The silica glass as described in the present application may be highpurity fused silica glass, undoped or doped at various levels.

As used herein, the term “low LIWFD” means a laser induced wavefrontdistortion, measured at 633 nm, of between −1.0 and 1.0 nm/cm whensubjected to 10 billion pulses of a laser operating at approximately 193nm having at a fluence of approximately 70 μJ·cm⁻¹ pulse⁻¹ and a pulselength of about 25 ns.

The process of the present invention is advantageous for making highpurity fused silica glass plate from silica glass tubes having circularstriae in cross-sections thereof perpendicular to the tube center axis.However, the process of the present invention is not limited to fusedsilica glass tubes. It may be adapted for reflowing other glass tubes aswell. Furthermore, glass tubes without striae in the tube cross-section,or with non-circular striae may be reflowed using the process of thepresent invention as well. That said, the present invention process isparticularly advantageous for thermal reflow of silica glass tube havingcircular striae to produce glass plates essentially free of observablestriae at least in a plane perpendicular to its optical axis.

Step (I) of the process of the present invention involves providing aready-to-flow notched glass tube having (a) a longitudinal tube centeraxis, (b) an identified section between two cross-sections perpendicularto the tube center axis having a section length L₁; and (c) a notch inthe direction of the tube center axis of the ready-to-flow notched glasstube through the tube wall. The ready-to-flow notched glass tube may beprovided and produced as such, or produced from a precursor glass tubewithout a notch.

In an embodiment of the latter case, step (I) comprises the followingsteps:

(Ia) providing a precursor glass tube having (a) a longitudinal tubeaxis, and (b) an identified section between two cross-sectionsperpendicular to the tube axis having a longitudinal section length L₁;and

(Ib) forming a notch in the direction of the tube axis of the precursorglass tube is through the tube wall, whereby the ready-to-flow notchedglass tube is formed.

The precursor glass tube concerned in the present application preferablytakes the shape of a cylinder having a longitudinal cavity therein. Theready-to-flow notched glass tube concerned in the present applicationpreferably takes the shape of a part of a cylinder having a longitudinalcavity and a longitudinal notch. The longitudinal tube axis and/or thelongitudinal center axis as discussed in the present application aretypically, but not limited to, the longitudinal axis of the outercylindrical surface of the tube, if the tube has a cylindrical outersurface. The longitudinal cavity within the tube is preferablycylindrical as well. It is desired that the cylindrical cavity and theouter cylindrical surface are concentric. However, as indicated in thegeneral description supra and the detailed description of the inventioninfra, they may be eccentric as well.

The precursor glass tube and the ready-to-flow notched glass tube may beproduced by conventional tube-making process, such as drawing, drillingof a glass rod, and the like. Drawing may be advantageously used forglasses with a relatively low softening temperature, such as normalsoda-lime glasses, borosilicate glasses, and the like. For glasseshaving a high softening temperature, such as fused silica glass, drawingmay be impractical, in which cases drilling may be advantageously used.

In the case of silica glass, particularly high purity synthetic silicaglass, and other high purity glass, the glass may be produced by knownvapor deposition processes, such as outside vapor deposition (“OVD”),inside vapor deposition (“IVD”), vapor axial deposition (“VAD”) frominorganic silicon precursor compounds, such as silicon halides, and/ororganosilicon precursor compounds, such as octamethylcyclotetrasiloxane(“OMCTS”), and the like. The glass may be doped or undoped. Theseprocesses may be plasma assisted as is known in the art. OVD, UVD andVAD are typically soot-to-glass processes in which soot preforms arefirst formed by silica soot particles generated by flame hydrolysis ofthe precursor compounds, which are in turn consolidated to formtransparent fused silica glass. In addition, as indicated infra, sol-gelprocess may be used for making synthetic silica glass as well.

In the case of OVD, silica soot preforms are formed on the outsidesurface of an axially rotating mandrel, which can be a solid core rod, atube, and the like, made of silica glass or other materials. The sootpreforms may be consolidated prior to the removal of the mandrel orthereafter. If the consolidation is performed prior to the removal ofthe mandrel, the consolidated silica glass generally has a differentcomposition from that of the mandrel. Thus the mandrel needs to beremoved—usually by drilling, and the like—to result in a glass tube thatcan be used as the precursor glass tube in the process of the presentinvention. If the consolidation is performed after the removal of themandrel, the consolidated glass directly forms a fused silica glasstube. These glass tubes, either formed from removing mandrel fromconsolidated glass, or from consolidating hollow soot preform withmandrel previously removed, may be used directly as the precursor glasstube in the process of the present invention. Alternatively, in certainsituations, as described infra it may be desirable to further process(such as reflow) the as-consolidated glass with mandrel remaining in thecenter before drilling to remove the mandrel, or the glass tube withmandrel removed, or the glass tube with a mandrel inserted therein,before the glass tubes are used as the precursor glass tube in theprocess of the present invention. Still alternatively, where a glasstube is used as the mandrel, the soot preform may be consolidatedwithout removing the mandrel. The thus formed glass tube with the innermandrel tube can be used as the precursor glass tube directly,optionally reflowed, cut to form the notch, then thermally reflowed toform the glass plate according to the process of the present invention.The glass tube mandrel thus forms at least a part of the surface part ofthe glass plate produced. The glass plate can then be ground to removethat surface part to result in a glass plate having essentiallyhomogeneous composition and property.

In the case of IVD, silica soot preforms are formed on the inner surfaceof an axially rotating tube, which can be made of silica glass or othermaterials. The soot preforms may be consolidated prior to the removal ofthe outside tube or thereafter. If the consolidation is performed priorto the removal of the tube, the consolidated silica glass generally hasa different composition from that of the outside tube. Thus the outsidetube can be removed after consolidation, with or without furtherprocessing (such as further thermal reflow such as, e.g., the SquashProcess described infra) to form the ready-to-flow silica glass tube ofthe present invention. Alternatively, the outside tube is retained afterconsolidation, during the formation of the precursor glass tube, duringthe formation of the notch and during the thermal reflow of the notchedglass tube. Thus after the thermal reflow process of the presentinvention, the outside tube forms the surface part of the glass plateproduced. The glass plate can then be ground to remove the surface partto result in a glass plate having essentially homogeneous compositionand property. If the consolidation of the soot preform is performedafter the removal of the outside tube, the consolidated glass forms afused silica glass tube having an essentially uniform composition. Thethus obtained glass tubes may be used directly as the precursor glasstube in the process of the present invention to form a notch thereon.Alternatively, in certain situations, as described infra it may bedesirable to further process (such as reflow by, e.g., the SquashProcess described below) the as-consolidated tube before it is used asthe ready-to-flow notched glass tube in the process of the presentinvention.

The VAD silica glass may be processed to form the ready-to-flow silicaglass tube according to the processes described above in connection withOVD and IVD mutatis mutandis.

It has been found that for synthetic silica glass made by the VAD, OVDand IVD processes, due to variations in the process conditions duringsoot deposition, variation in composition in different layers of thesoot preform can occur. Such composition variation, typically largelycircular, can lead to striae upon consolidation. For the purpose of thepresent invention, “striae” mean variations in the bulk in theconsolidated glass in composition and/or physical properties(particularly refractive index) with magnitudes that are detrimental tothe performance of the glass for its intended purpose. In a given areaof a given plane, striae may appear in a repeated pattern at certainfrequency, or may occur sporadically. Striae, especially those in theform of refractive index variation, are highly undesirable, particularlyif present in a plane perpendicular to the optical axis of an opticalmember. As described supra, visible striae in planes perpendicular tothe optical axis may be present, if glass plates are formed directlyfrom pressing the cylindrical silica glass tubes having circular striaein its cross-sections perpendicular to the tube axis.

The vapor deposition processes mentioned above were previously used inthe art in producing optical waveguide preforms. Thus preforms typicallyhave a relatively long length and small diameter. Thus silica glasstubes directly made from these waveguide preforms (such as by removingthe mandrel) tend to have a relatively long length and small tube wallthickness. As mentioned supra, these slim tubes made from theas-consolidated silica glass may be directly used as the precursor glasstubes in the process of the present application. However, for aplurality of end applications of the silica glass, the resultingreflowed glass plate would not have sufficient width or thickness. Forexample, the production of optical blanks for regular photomasksubstrates and/or lens elements used in modern photolithographyoperating at about 248 and 193 nm by using the thermal reflow process ofthe present invention requires the ready-to-flow notched glass tube havea thicker tube wall and larger tube outer diameter.

The present inventors have devised a method by which slim fused silicacylinders or tubes of the dimension of optical waveguide preforms can beformed into silica glass tubes having higher tube wall thickness andlarger tube outer diameter suitable for the production of optical blanksfor use as regular photomask and optical element in deep UV and vacuumUV photolithography. This method is referred to as the “Squash Process”hereinafter. In general terms, the Squash Process comprises, in step(Ia) of the process of the present invention mentioned above, thefollowing steps:

(I0) providing a precursor glass cylinder having a precursor cylinderaxis, a length L₀ in the direction of the precursor cylinder axis and aprecursor cylinder outer diameter OD₀;

(I1) thermally reflowing, with optional pressing, the precursor glasscylinder; and

(I2) optionally drilling in a direction essentially parallel to theprecursor cylinder axis to form a cylindrical inner cavity,

whereby the precursor glass tube is formed to have a longitudinal tubeaxis, an outer diameter OD₁ and a length L₁ in the direction of the tubeaxis. It is preferred the tube axis is essentially parallel to or thesame as the precursor cylinder axis of the precursor glass cylinder,L₁<L₀, and OD₁>OD₀.

In the preferred embodiment of the Squash Process, 0.3L₀≦L₁≦0.8L₀. Thusas a result of the Squash Process, the length of the glass cylinder isreduced.

In the Squash Process, it is preferred that:

in step (I0), the precursor glass cylinder comprises an inner glass canehaving the same or differing composition and/or properties than theglass surrounding the inner glass cane; said inner glass cane is locatedapproximately at the center of the precursor glass cylinder and has adiameter of ID₀;

in step (I2), the inner glass cane is essentially completely removed.

In this preferred embodiment of the Squash Process, it is furtherpreferred that after step (I2), the precursor glass tube has an innercylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁. Thus thewall thickness of the precursor glass tube is higher than that of theprecursor glass cylinder if the inner glass cane had been removed fromthe precursor glass cylinder.

In another embodiment of the Squash Process, in step (I0), the providedprecursor glass cylinder has an inner cylindrical cavity the axis ofwhich is parallel to the precursor cylinder axis, and the innercylindrical cavity has a diameter of ID₀. Preferably, the innercylindrical cavity has a diameter ID₁, and OD₀−ID₀<OD₁−ID₁. Thus thewall thickness of the resulting precursor glass tube is higher than thatof the precursor glass cylinder (which is actually a slimmer tube).

It has been found that in order to produce glass plates having a largerthickness, usually higher thickness of the tube wall of theready-to-flow notched glass tube is desired.

FIG. 1 schematically illustrates the steps of an embodiment of theSquash Process according to the present invention. In this figure, aprecursor glass cylinder A2 is provided in step (I0) from a cylinder A1.The precursor glass cylinder has an outer diameter OD₀, a precursorlongitudinal cylinder axis and a length L₀ in the direction of thecylinder axis. In addition, the precursor glass cylinder A2 has an innercavity or mandrel having the shape of a cylinder with an inner diameterof ID₀. In this figure, it is shown that the precursor glass cylinder A2are concentric with the mandrel or inner cavity. In practice, they maybe eccentric. In step (I1), the precursor glass cylinder is reflowed toform a new cylinder having a shorter length L₁ and a larger outerdiameter OD₁. During the reflow process, the inner cavity or the mandrelis deformed. Thus in step (I2), the mandrel is essentially completelydrilled out, or the inner cavity is further drilled to redress thedeformation thereof, such that a new inner cylindrical cavity having adiameter of OD₁ is formed. Alternatively, if A2 has an inner cavityinstead of a mandrel, a deformable or non-deformable mandrel (made ofsilica, graphite, or other materials) may be inserted into the innercavity during step (I1), such that at the end of (I1), the mandrel canbe removed, with or without the additional step (I2), to result in theprecursor glass tube. The outer cylindrical surface having a diameterOD₁ and the inner cylindrical surface having a diameter ID₁ togetherwith the two end cross-sectional surfaces define the precursor glasstube A as provided in step (I) of the process of the present invention.As is obvious from the figure, because L₁<L₀, the wall thickness of theprecursor glass tube A is larger than that of the precursor glasscylinder A2.

FIG. 10 schematically illustrates the cross-sectional view of a furnacedevice in which step (I1) of the Squash Process can be implemented. Inthis figure, the device 1001 comprises an outer barrel 1013 in which acrucible 1009, and a sleeve 1003 are placed. The sleeve is connected tothe outer barrel 1013 via supporting arms 1011. The outer barrel 1013 issupported on ringwall 1015. Both the sleeve and the crucible are made ofrefractory materials, such as purified graphite. The crucible has adepth of H, and an inner diameter of MD. The precursor glass cylinderhaving an outer diameter of OD₀ and a length L₀ is first placed on thebottom plate of the crucible 1009 and partly inside the sleeve 1003. Inthis figure it is also shown the optional weight 1005 placed atop theprecursor glass cylinder. The furnace is then heated to a reflowtemperature, such as above the softening point of the glass, where theglass reflows under the influence of its own gravity and pressed by theweight 1005. The sleeve 1003 guides the reflow of the glass. At the endof the reflow process, a glass cylinder having an outer diameter OD₁ anda length L₁ is obtained, where OD₀<OD₁≦MD, and L₀<L₁≦H. The thus formednew cylinder typically has a center axis of the precursor glass cylinderbecause of the use of the guiding sleeve 1003. In addition, if theprecursor glass cylinder has essentially circular striae incross-sections perpendicular to the center axis of the precursor glasscylinder, after the thermal reflow, in the new glass cylinder, thestriae will be maintained largely in circular shape in cross-sectionsperpendicular to the center axis of the new cylinder.

By virtue of the Squash Process, glass plate with larger width andhigher thickness may be produced from slim glass tubes and rods.

In step (Ib) of the process of the present invention, a notch is cut inthe precursor glass tube in the direction of, preferably parallel to,the longitudinal tube center axis of the precursor glass tube throughthe tube wall.

FIG. 4 illustrates the cross-section 401 of the precursor glass tubeaccording to an embodiment of the process of the present invention. Theprecursor glass tube in this figure has an outer diameter ID₁, an innercavity having a diameter ID₁, and a plurality of circular striae 403.FIGS. 5, 6, 7 and 8 illustrate the geometries of various notches thatcan be formed into the precursor glass tube of FIG. 4. FIG. 5 shows anotch 501 having an essentially rectangular cross-section with a widthS<ID₁. Thus the notch and the inner cavity together form a key-holegeometry. FIG. 6 shows a notch 601 having an essentially rectangularcross-section with a width S≈ID₁. FIG. 7 shows a trapezoidal (truncatedV-shaped) notch 701 having a short base <ID₁. FIG. 8 shows anothertrapezoidal notch 801 with a short base approximating ID₁. As indicatedby the dotted curve lines, the cross-sections of all the notches inthese figures have an outer arc (503, 603, 703 and 803 in FIGS. 5, 6, 7and 8, respectively) with a length of L_(arc). All these notches can beproduced and used in the process of the present invention.

Preferably, the notch formed in the wall of the ready-to-flow notchedglass tube has a center plane passing through the longitudinal tubecenter axis of the ready-to-flow notched glass tube, and the two sidesof the notch beside the center plane are essentially symmetric. In suchscenario, if the outer cylinder and the center cylindrical cavity of theready-to-flow notched glass tube are concentric, the notch may be formedat any location of the circumference of the tube wall; if the outercylinder and the center cylindrical cavity of the ready-to-flow notchedglass tube are eccentric, the notch is preferably formed at the locationwhere the center plane of the notch passes the maximal or minimalthickness, preferably the minimal thickness, of the precursor glasstube. Thus, it is preferable that the two sides of the notchedready-to-flow notched glass tube about the center plane of the notch aresymmetric.

FIG. 2 schematically illustrates the notch-formation step according tothe process of the present invention. The notch illustrated in thisfigure corresponds to that of FIG. 5, where the width of the notch is S,and S<ID₁.

The notch can be formed by various methods and equipment known in theart, such as by using wire saw, water jet, band saw, and the like.Preferably, after cutting the notch, the notched glass tube isthoroughly cleaned before performing step (III) of the preset invention.Such cleaning may include acid (HCl, HF, and the like) washing, solventwashing, Cl₂ treatment at high temperature, and the like, so thatcontamination introduced by the cutting process is eliminated orminimized.

Step (II) of the process of the present invention comprises thermallyreflowing the ready-to-flow notched tube thus provided in step (I) at anelevated temperature such that the notched tube reflows to form a glassplate. The formed glass plate preferably has two major surfaces and anoptical axis essentially perpendicular to the two major surfaces.Generally this step is conducted with the notched side and the notchfacing upwards and the un-notched side placed on the surface of asupport, such as the bottom plate of a crucible. It is preferred thatthe notch is placed in an essentially vertical position.

This thermal reflow step (III) is advantageously performed at above thesoftening point of the glass. For fused silica glass whose softeningtemperature is about 1650° C., this step is usually carried out at above1700° C., but below 2000° C., preferably below 1900° C.

If high purity of the glass and a low metal contamination are requiredfor the glass, which is the case for high purity synthetic silica glassfor use in deep UV and vacuum UV lithography, it is desired that thestep (II) is performed in a purifying atmosphere comprising a cleansinggas. The cleansing gas may be, for example, a halogen, ahalogen-containing compound and compatible mixtures thereof. Suchhalogen-containing compound may be selected from HX, C_(a)S_(b)X_(c) andcompatible mixtures thereof, where X is selected from F, Cl and Br, a, band c are non-negative integers meeting the valency requirements of theindividual elements.

FIG. 3 schematically illustrates step (II) of an embodiment of theprocess according to the present invention. In this figure, theready-to-flow notched glass tube B is reflowed and extended sideways toform a glass plate C. The plate C is placed in a three-dimensionalorthogonal coordinate system xOyz. The resultant glass plate C has twoessentially flat major surfaces: a smaller upper surface with a width L₃(shown above plane xOy) and a larger lower surface with a width L₄(shown in plane xOy). Both surfaces have a length of L₂. The axis z isthe optical axis of the glass plate. The larger surface having an areaL₂-L₄ essentially corresponds to the outer cylindrical surface of theready-to-flow notched glass tube B, and the smaller surface having anarea L₂·L₃ essentially corresponds to the inner cylindrical surface ofthe ready-to-flow notched glass tube B. The thickness T of the resultantglass plate C corresponds to the wall thickness 0.5(OD₁−ID₁) of theready-to-flow notched glass tube B. The plate having dimension of L₂·L₃T represents the useable plate that can be extracted from the reflowedglass body. Typically, T<0.5 (OD₁−ID₁). Typically, L₃>π·ID₁, which meansthat the inner cylindrical cavity surface is stretched during the reflowprocess. FIG. 3 shows the edge portion of the reflowed glass plate ashaving a part protruding upwards. In practice, the edges may have adifferent configuration, depending on the shape and dimension of theready-to-flow notched glass tube, the notch, the reflow temperature andtime.

If the ready-to-flow notched glass tube has essentially circular striaesuch as those illustrated in FIG. 4, in step (II), such striae arenormally reoriented, extended and may be twisted slightly. FIG. 9schematically illustrates the cross-section of a useable glass plateextracted from the plate showed in FIG. 3. This figure shows the remnantstriae in the plate when viewed from the direction of axis y of theplate in FIG. 3, which are essentially parallel to each other, butextend in directions essentially perpendicular to the optical axis z.Thus the circular striae of the ready-to-flow notched glass tube isreflowed and reoriented in the glass plate. The overall result is, whenviewed in the direction of the optical axis of the resultant glass plate(axis z), essentially no striae is observable. Surprisingly, it has alsobeen found that in certain preferred embodiments, even if the startingglass tube has circular striae as illustrated in FIG. 4, the resultantglass plate may still be devoid of striae when viewed at least in onedirection perpendicular to the optical axis of the plate. It ishypothesized by the present inventors that, in practice, the reorientedstriae may not be strictly parallel to each other. However, because thetube walls are stretched during the thermal reflow of the presentinvention, the dimensions of the striae are reduced. Moreover, the finalstriae in the glass plate may curve and twist slightly, leading tomutual cancellation of distortion caused by each other. Thus, theoverall effect is reduced striae in the resultant plate and improvedoptical performance at least in the direction of the optical axis.

As a result of step (II), usually L₁≦L₂≦2L₁, preferably L₁≦L₂≦1.5L₁,more preferably L₁≦L₂≦1.2L₁. Thus, at the end of the thermal reflowprocess of the present invention, the length of the ready-to-flownotched glass tube has been extended. However, it is preferred that thelength is not significantly extended, especially where a high thicknessof the final glass plate is desired. As discussed infra where thethermal reflow of the present invention is performed without additionalexternal mechanical assistance, the reflow is essentially the result ofthe influence of the tube gravity on the notched glass tube. Even ifmechanical assistance is adopted, it is generally preferred that theoverall effect of the mechanical assistance is similar to the effect ofthe gravity. It is generally preferred that the thermal reflowtemperature is not overly high such that the viscosity of the glassbecomes so low that the glass flows freely in all directions. Rather, itis preferred that the reflow temperature is controlled such that themovement of the notched glass tube is mostly limited to sidewaysroll-out during a desired roll-out time period. Hence the preferencethat L₁≦L₂≦1.5L₁, more preferably L₁≦L₂≦1.2L₁.

According to the process of the preset invention, it is preferred thatin the resultant glass plate, L₃≦0.5·L₄, preferably L₃≧0.8L₄, morepreferably L₃≧0.9L₄, still more preferably L₃≧0.95L₄. As indicated suprathe plate having dimension L₂·L₃·T represents the useable part for theintended purpose that can be produced from the reflowed glass plate atthe end of step (II) of the process of the present inventioncorresponding to the identified section of the ready-to-flow notchedglass tube provided in step (I) as mentioned above. This would allow ahigher yield of the final useable glass. Typically, the edge portions(illustrated as enclosed by the dotted edge line and the side line ofthe useable rectangular plate, 903 in FIG. 9) of the reflowed glassplate tend to have less compositional and/or property homogeneity thanthose in the flat useable part at least when viewed in the direction ofthe optical axis of the plate. Thus they may need to be sacrificed whenextracting the useable part from the reflowed glass.

In a preferred embodiment of the process of the present invention, theready-to-flow notched glass tube and its inner cavity are bothcylindrical and have a diameter of OD₁ and ID₁, respectively, and instep (II), the identified section of the ready-to-flow notched glasstube is formed into a glass plate having two essentially flat majorsurfaces, a width of a first major flat surface of L₃, a width of asecond major surface of L₄, L₄≧L₃, a length of both major surfaces ofL₂, and a thickness between the two essentially flat major surfaces ofT. It is preferred in this embodiment thatπ·OD₁−L_(arc)≦L₄≦2(π·OD₁−L_(arc)), preferablyπ·OD₁−L_(arc)≦L₄≦1.8(π·OD₁−L_(arc)) more preferablyπ·OD₁−L_(arc)≦L₄≦1.5(π·OD₁−L_(arc)), where L_(arc) is the length of theouter arc of the cross-section of the notch formed on the tube wall.Thus the outer cylindrical surface of the ready-to-flow notched glasstube is preferably stretched to a desired level during the reflowprocess. Typically, in order to obtain a thicker glass plate, it isdesired that the ratio of L₄/(π·OD₁−L_(arc)) is closer to 1. Asmentioned above, during the thermal reflow process of the presentinvention, the inner cavity surface is stretched. Usually, the smallerthe ratio of the diameter of the inner cavity to the diameter of theouter cylinder, ID₁/OD₁, the higher the extent to which the inner cavitysurface is stretched. Nonetheless, to obtain a glass plate with largerarea, it is preferred that L₃≧1.5π·ID₁, more preferably L₃≧2π·ID₁, stillmore preferably L₃≧3π·ID₁. Further, as discussed supra, because of thestretch, the thickness of the resultant glass plate in step (II), T,tends to be smaller than the wall thickness of the ready-to-flow notchedglass tube in the process of the present invention. Nonetheless, it ispreferred that 0.10·(OD₁−ID₁)≦T≦0.45·(OD₁−ID₁), more preferably 0.10·(OD₁−ID₁)≦T≦0.40·(OD₁−ID₁), and still more preferably0.10·(OD₁−ID₁)≦T≦0.30·(OD₁−ID₁).

Crucibles made of purified graphite are preferred for step (II) if fusedsilica glass is the material of the ready-to-flow notched glass tube.Purified porous ceramic felt liner may be used in conjunction. Theceramic felt liner can be fibrous Zirconia felt (such as ZYF-100manufactured by Zircar of Florida NY). The use of the ceramic feltinhibits reaction between the glass and graphite at high processingtemperatures. This material is seen to be non-reactive and non-wettingwith fused silica as well as allowing for escape of gaseous speciesthrough the felt. The latter feature avoids entrainment of gases intothe glass during reflow and roll-out. Alternative crucible and linermaterials can be employed depending on furnace environment and degree towhich it may react with and be wetted by fused silica glass. Coating ofcrucible and liner materials can be employed to minimize the reactionwith fused silica glass as well as potential for contamination of theglass. Materials identified to date include refractory metals such asmolybdenum and tungsten, ceramics including stabilized zirconia,zirconium silicate (Zircon), silicon carbide, alumina, andcrucible/liner coating materials such as boron nitride, yttrium oxide,and carbon. It should be noted that the use of a rigid liner orsubstrate material in contact with the glass during roll out will alsoimprove the resultant blank homogeneity versus that attained using thecompliant zirconia felt liner. Special wall design may be employed aswell.

Once positioned in the crucible and loaded into the furnace the glass isheated to reflow/roll-out temperature to induce softening and stretchingof the glass (i.e., roll-out). For silica glass, the maximum furnacetemperature is on the order of 1700° C. to 1900° C. Current experimentalresults indicate that it is desired to raise the furnace temperature toabout 1800° C. to 1850° C. with hold times of up to 1 hour for fusedsilica glass with nominal β-OH concentrations up to 500 ppm by weight.Experimentation to date has shown heating rates of 50° C./hour to 600°C./hour above the glass anneal point to the maximum temperature usefulfor roll out. Higher ramp rates (e.g., 180° C./hour to 600° C./hour) areseen to be more effective in stretching the center portion of the glassduring roll out. At temperatures between the anneal point and glassdevitrification range a hold at temperature can be employed to yieldmore uniform temperature through the glass blank. Present results wereattained using graphite resistively heated furnaces with inert gasatmospheres of helium or argon employed during thermal treatment.Pressures of ˜1 to 3 psi (˜6.89×10³ to 2.07×10⁴ Pa) above atmosphericpressure were maintained during the thermal cycle. The roll-out processis not seen to be restricted the above type of furnace design orenvironment. Alternative furnace types and atmospheres can be employedfor this process so long as materials used for crucible and linermaterials are compatible with glass, furnace materials and environment.

The roll out process has been seen to be effective for silica glassblanks with wall thicknesses between 1″ to 2.5″ (2.5 to 6.3 cm)experimentally. Modeling analysis indicates that notched blanks withboth thinner and thicker wall thickness can be rolled out. The rollprocess has also been seen to be scalable in experimental trials forblank lengths between 4″ to 10″ (10 to 25 cm). No limit seen ispresently for the maximum length roll out possible other than furnacesize restrictions.

A study of the impact of roll out on index homogeneity was conducted fora sample. This sample was a key-holed notched blank having a key holegeometry illustrated in FIG. 5. Index homogeneity measurements in aplane perpendicular to the optical axis, post grind and anneal indicatesa Δn of <3 ppm over a 127 mm clear aperture at a final ground blankthickness of 31.8 mm. Additionally, no micro-striae were observed in thesame plane.

In step (II) of the process of the present invention, external forcesother than gravity of the tube may be exerted on the ready-to-flownotched glass tube, such as on the two side surfaces of the notch or tothe surface of the inner cavity, to facilitate the reflow of the glass.Such mechanical assistance of the roll-out or reflow process canexpedite the reflow process or allow the roll-out to be carried out at alower temperature. FIG. 11 schematically illustrates a ready-to-flownotched glass tube of FIG. 7 further equipped with stretching arms 1101.During the roll-out process, external force F is applied to both notchsurfaces via the arms. Alternative methods for mechanically assistedroll out are possible as well.

FIGS. 12 and 13 illustrate an alternative approach to mechanicallyassisted roll-out. In FIG. 12, a notched glass tube having essentiallythe configuration of FIG. 5 is placed on a mandrel 1203 inserted throughthe inner cavity of the tube. The notch on the glass tube wall is placedsideways. A setter 1201 is placed atop the upper part of the glass tubeabove the notch. The entire set-up is heated to an elevated temperatureto allow the lower part of the tube to roll out to an essentiallyvertical position to form the essentially straightened part 1205. InFIG. 13, the partially rolled-out piece of glass is placed on a slope1301, where the un-rolled-out part of the tube 1207 is allowed to rollout. Thus at the end of the roll-out process, an essentially flat glassplate 1309 is formed on the slope. In FIG. 13, a mandrel 1305 is alsoillustrated. An external force F is applied to the partially rolled-outglass piece via the mandrel. At the end of the roll-out process, themandrel 1305 is placed into a receptive notch 1307 formed on the sidewall 1303.

FIG. 14 illustrates another embodiment of mechanically assisted roll-outprocess of the present invention. In this embodiment, a glass tubehaving essentially the configuration of FIG. 7 is rolled out via theassistance of an articulating mandrel 1401 and a plunger 1403. Duringthe initial stage of the roll-out, the glass tube is essentially pressedopen via the assistance of the mandrel and the plunger. Subsequently,the partially rolled-out tube having a larger opening is allowed toreflow to substantially flat as described above. The total roll-out timecan be shortened significantly by using mechanical assistance.

Surprisingly, the thermal reflow process of the present invention canresult in a glass plate with a birefringence map in which the fast axisdirections of the measured pixels have a low randomness factor. Thus,preferably, the process of the present invention is characterized by:

in step (II), the identified section of the ready-to-flow notched glasstube forms an identified glass plate having two essentially flat majorsurfaces, a width of the first major flat surface of L₃, a width of asecond major surface of L₄, L₄≧L₃, a length of both major surfaces ofL₂, and a thickness between the two essentially flat major surfaces ofT; and

measured in a plane perpendicular to the optical axis of the identifiedglass plate, the identified glass plate with a surface area of aboutL₃·L₂ upon edge removal, surface lapping and annealing has abirefringence pattern in which fast axis directions have a randomnessfactor of between −0.50 and 0.50, preferably between −0.40 and 0.40,more preferably between −0.30 and 0.30.

The glass-plate making process of the present invention has, inter aliathe following advantages:

(1) The process allows cylindrical fused silica blanks of thick wallcross section to be used for manufacture of parts with plate and/ordisc-like geometry for use in photolithography lens applications. Theprocess is scalable.

(2) The process of the present invention can be carried out without theuse of elaborate fixture in step (II). That is, the roll-out can beeffected via the influence of gravity. However, as described supra, itis not ruled out that the roll-out is carried out with mechanicalassistance.

(3) The process of the present invention can provide flat planar partswhich do not require secondary thermal processing steps for substantialleveling or straightening.

(4) The process of the present invention is capable of providing platesof width significantly larger than initial blank diameter.

(5) Circular striae in terms of compositional variation (such as OHconcentration variation) and/or property variation (such as refractiveindex variation) can be realigned such that they do not interfere withthe optical performance of resultant glass plate without removing thestriae via complex homogenization and mixing of the glass.

(6) Provides means to process OVD blanks with center core removedavoiding the issues related to the index inhomogeneity seen at thecore-overclad interface.

The Squash Process described summarily and in detail above for reforminga glass cylinder constitutes a second aspect of the present invention.

A third aspect of the present invention is thus a synthetic silica bodyhaving an optical axis and a birefringence pattern as measured in aplane perpendicular to the optical axis in which fast axis directionshave a randomness factor of between −0.50 and 0.50, preferably between−0.40 and 0.40, more preferably −0.30 and 0.30, still more preferablybetween −0.20 and 0.20. Preferably, the synthetic silica body is a platehaving two essentially flat and essentially parallel major surfaces,each major surface having an area of at least 1 cm², preferably at least4 cm², more preferably at least 16 cm². In certain embodiments, each ofthe major surfaces has an area of at least 100 cm². In otherembodiments, each of the major surfaces has an area of at least 225 cm²,such as about 400 cm², 625 cm², 900 cm², or even larger. Preferably, thesynthetic silica body has a refractive index variation Δn as measured ina plane perpendicular to the optical axis, wherein Δn≦10 ppm, preferablyΔn≦5 ppm, more preferably Δn≦1 ppm, most preferably Δn≦0.5 ppm.Preferably, the synthetic silica glass body of the present invention hasan internal transmission at about 193 nm of about 99.65% cm⁻¹, morepreferably at least 99.70% cm⁻¹, still more preferably at least 99.75%cm⁻¹, still more preferably at least 99.80% cm⁻¹, most preferably atleast 99.85% cm⁻¹. Preferably, the synthetic silica body of the presentinvention has a low level of LIWFD. Preferably, the synthetic silicabody of the present invention has a birefringence of less than 5 nm/cm,preferably less than 3 nm/cm, more preferably less than 1 nm/cm, mostpreferably less than 0.5 nm/cm, when measured in a plane perpendicularto the optical axis. Preferably, the synthetic silica body of thepresent invention has a fictive temperature of lower than 1150° C.,preferably lower than 1050° C., more preferably lower than 1000° C.,most preferably lower than about 900° C.

The birefringence of a glass body, such as a fused silica blank, isusually analyzed by dividing the glass body aperture into an array ofpixel elements and then using a polarimeter to measure the magnitude andfast axis direction of each pixel element. The “randomness” of thebirefringence fast axis directions can be assessed either with a simplevisual evaluation of the birefringence map or through equations whichaverage the direction of the birefringence over the clear aperture ofthe glass body. It has been found that, as shown in FIG. 15, certainsilica glass plates exhibit a tangential pattern, meaning that amajority of the glass volume has fast birefringence axes which areperpendicular to radial lines. With this birefringence profile, thedirection vectors produce a “tree ring” pattern. As shown in FIG. 16,certain other silica glass plates exhibit a radial pattern, meaning thata majority of the glass volume has fast birefringence axes which areparallel to any radial line. With this birefringence profile thedirection vectors produce a “star burst” pattern. FIG. 17 shows the fastbirefringence axis pattern of a glass produced using the roll-out reflowprocess of the present invention. Its direction vectors show a mixedpattern in which no particular orientation is dominant.

As used herein, the randomness factor of fast axis directions (FR) iscalculated from a birefringence map as follows:${FR} = \frac{\sum\left\lbrack \left. {{\cos\left( {\theta - \gamma} \right.} - {{\sin\left( {\theta - \gamma} \right)}}} \right\rbrack \right.}{N}$where:

θ is the angle of the pixel on the measured glass body in sphericalcoordinates;

γ is the orientation angle of the fast axis of the measuredbirefringence in the pixel;

N is the number of pixels measured in the aperture; and

the operator |x| means the absolute value of x.

The FR as so defined ranges between −1 and 1. When it is equal to −1,the fast axis profile is tangential (“tree rings” pattern). When it isequal to +1, the pattern is radial (“star burst” pattern). A value ofzero represents complete randomness of the direction of the fast axes ofthe birefringence in those measured pixels. As an example, applying thisformula to the fast axis maps given above generates the followingvalues: FIG. No. FR Visual Birefringence Pattern 15 −0.88 tangential 16+0.93 radial 17 −0.36 mixed

A fourth aspect of the present invention is an optical element having anoptical axis which is made from the synthetic silica body describedsupra. Preferably, the optical axis of the optical element is parallelto the optical axis of the synthetic silica body. In a preferredembodiment, the optical element is a lens element for use inlithographic device operating in deep or vacuum UV wavelength regions,such as about 248 nm, 193 nm and shorter. In another preferredembodiment, the optical element is a photomask substrate for use inlithographic devices, such as those operating in deep or vacuum UVwavelength regions, such as at about 248 nm, 193 nm and shorter. Inother embodiments, the optical element of the present invention can beused in laser generators, sputter targets, mirrors, optical inspectingdevices, and the like.

It will be apparent to those skilled in the art that variousmodifications and alterations can be made to the present inventionwithout departing from the scope and spirit of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A synthetic silica body having an optical axis and a birefringencepattern as measured in a plane perpendicular to the optical axis inwhich fast axis directions of the measured birefringence have arandomness factor of between −0.50 and 0.50.
 2. A synthetic silica bodyaccording to claim 1, which is a plate having two essentially flat andessentially parallel major surfaces, each major surface having an areaof at least 1 cm².
 3. A synthetic silica body according to claim 1having an internal transmission at about 193 nm of at least 99.65% cm⁻¹.4. A synthetic silica body according to claim 1 having a low level ofLIWFD.
 5. A synthetic silica body according to claim 1 having abirefringence of less than 5 nm/cm, when measured in a planeperpendicular to the optical axis.
 6. A synthetic silica body accordingto claim 1, having a fictive temperature of lower than 1150° C.
 7. Asynthetic silica body according to claim 1 essentially free of striaewhen viewed in the direction of the optical axis.
 8. A synthetic silicabody according to claim 7 essentially free of striae when viewed in atleast one additional direction perpendicular to the optical axis.
 9. Anoptical element having an optical axis which is made from the syntheticsilica body according to claim 1, wherein the optical axis of theoptical element is parallel to the optical axis of the synthetic silicabody.
 10. An optical element according to claim 9 which is a lenselement for use in lithographic device operating in deep or vacuum UVwavelength region.
 11. A process for making glass plate comprising thefollowing steps: (I) providing a ready-to-flow notched glass tube having(a) a longitudinal tube center axis, (b) an identified section betweentwo cross-sections perpendicular to the tube center axis having alongitudinal section length L₁; and (c) a notch in the direction of thetube center axis of the ready-to-flow notched glass tube through thetube wall; and (II) thermally reflowing the ready-to-flow notched glasstube thus formed in step (I) at an elevated temperature to form a glass.12. A process in accordance with claim 11, wherein in step (II), thenotched side and the notch of the glass tube face upwards and theun-notched side is placed on the surface of a support.
 13. A process inaccordance with claim 111, wherein the glass tube has striae when viewedin the direction of the tube center axis.
 14. A process in accordancewith claim 13, wherein the glass tube has essentially circular striaewhen viewed in the direction of the tube center axis.
 15. A process inaccordance with claim 11, wherein the glass is consolidated fusedsilica.
 16. A process in accordance with claim 15, wherein the silicaglass is produced by outside vapor deposition.
 17. A process inaccordance with claim 15, wherein the silica glass is produced by insidevapor deposition.
 18. A process in accordance with claim 15, wherein theglass is high purity consolidated fused silica and step (II) isconducted in the presence of a purifying atmosphere comprising acleansing gas.
 19. A process in accordance with claim 18, wherein thecleansing gas comprised in the purifying atmosphere is selected from F₂,Cl₂, Br₂, a halogen-containing compound, and compatible mixturesthereof.
 20. A process in accordance with claim 11, wherein in step (I),the notch is formed to have a center plane passing through the tubecenter axis of the ready-to-flow notched glass tube, and the two sidesof the notch beside the center plane are essentially symmetric.
 21. Aprocess in accordance with claim 11, wherein in step (I), the notch isformed to have two essentially parallel sides.
 22. A process inaccordance with claim 11, wherein in step (I), the notch is formed tohave an essentially truncated “V” shape cross-section when cut by aplane perpendicular to the tube center axis of the ready-to-flow notchedglass tube.
 23. A process in accordance with claim 11, wherein in step(I), the provided ready-to-flow notched glass tube has a cross-sectionthat is part of a ring-shape defined by an essentially circular outerboundary having a diameter of OD₁ and an essentially circular innerboundary having a diameter of ID₁ when cut by a plane perpendicular tothe center axis of the tube.
 24. A process in accordance with claim 23,wherein in step (I), the outer boundary and the inner boundary of thering-shape are essentially concentric.
 25. A process in accordance withclaim 23, wherein in step (I), the outer boundary and the inner boundaryof the ring shape are essentially eccentric.
 26. A process in accordancewith claim 25, wherein in step (I), the notch is formed at the locationsuch that the center plane of the notch is where the thickness of thewall of the ready-to-flow notched glass tube is essentially the minimal.27. A process in accordance with claim 11, wherein in step (II), theidentified section of the ready-to-flow notched glass tube is formedinto a glass plate having two essentially flat major surfaces, a widthof a first major flat surface of L₃, a width of a second major surfaceof L₄, L₄≧L₃, a length of both major surfaces of L₂, and a thicknessbetween the two essentially flat major surfaces of T.
 28. A process inaccordance with claim 27, wherein L₁≦L₂≦2L₁.
 29. A process in accordancewith claim 27, wherein L₃≧0.5 L₄.
 30. A process in accordance with claim23, wherein in step (II), the identified section of the ready-to-flownotched glass tube is formed into a glass plate having two essentiallyflat major surfaces, a width of a first major flat surface of L₃, awidth of a second major surface of L₄, L₄≧L₃, a length of both majorsurfaces of L₂, and a thickness between the two essentially flat majorsurfaces of T.
 31. A process in accordance with claim 30, whereinπ·OD₁−L_(arc)≦L₄≦2(π·OD₁−L_(arc)), where L_(arc) is the outer arc lengthof the notch.
 32. A process in accordance with claim 30, whereinL₃≧1.0π·ID₁.
 33. A process in accordance with claim 30, wherein 0.10·(OD₁−ID₁)≦T≦0.45·(OD₁−ID₁).
 34. A process in accordance with claim 11,wherein: in step (II), the identified section of the ready-to-flownotched glass tube forms an identified glass plate having twoessentially flat major surfaces, a width of the first major flat surfaceof L₃, a width of a second major surface of L₄, L₄≧L₃, a length of bothmajor surfaces of L₂, and a thickness between the two essentially flatmajor surfaces of T; and measured in a plane perpendicular to theoptical axis of the identified glass plate, the identified glass plateupon edge removal and surface lapping with a surface area of about L₃·L₂has a birefringence pattern in which fast axis directions have arandomness factor between −0.50 and 0.50.
 35. A process in accordancewith claim 11, wherein step (I) comprises the following steps: (Ia)providing a precursor glass tube having (a) a longitudinal tube axis,and (b) an identified section between two cross-sections perpendicularto the tube axis having a longitudinal section length L₁; and (Ib)forming a notch in the direction of the tube axis of the precursor glasstube through the tube wall, whereby the ready-to-flow notched glass tubeis formed.
 36. A process in accordance with claim 35, wherein the glassis silica and step (Ia) comprises the following steps: (Ia1) forming asilica soot preform by the OVD process on a mandrel; (Ia2) consolidatingthe silica soot preform into fused silica glass without previouslyremoving the mandrel; and (Ia3) removing the mandrel to form theprecursor glass tube.
 37. A process in accordance with claim 35, whereinthe glass is silica and step (Ia) comprises the following steps: (Ia1)forming a silica soot preform by the OVD process on a mandrel; (Ia2)removing the mandrel from the soot preform; and (Ia3) consolidating thesilica soot preform into fused silica glass, whereby the precursor glasstube is formed.
 38. A process in accordance with claim 35, wherein theglass is silica; and step (Ia) comprises the following steps: (Ia1)forming a silica soot preform by the OVD process on a glass tubemandrel; (Ia2) consolidating the silica soot preform into fused silicaglass without previously removing the mandrel, whereby the precursorglass tube is formed.
 39. A process in accordance with clam 38comprising the following step (III) after step (II): (III) removing thesurface part of the glass plate resulting from the glass tube mandrel.40. A process in accordance with claim 35, wherein the glass is silicaand step (Ia) comprises the following steps: (Ia1) forming a silica sootpreform by the IVD process on the inner surface of an outside tube;(Ia2) consolidating the silica soot preform into fused silica glasswithout previously removing the outside tube; and (Ia3) removing theoutside tube to form the precursor glass tube.
 41. A process inaccordance with claim 35, wherein the glass is silica and step (Ia)comprises the following steps: (Ia1) forming a silica soot preform bythe IVD process on the inner surface of an outside tube; (Ia2) removingthe outside tube from the soot preform; and (Ia3) consolidating thesilica soot preform into fused silica glass, whereby the precursor glasstube is formed.
 42. A process in accordance with claim 35, wherein theglass is silica and step (Ia) comprises the following steps: (Ia1)forming a silica soot preform by the IVD process on the inner surface ofan outside tube; and (Ia2) consolidating the silica soot preform intofused silica glass without previously removing the outside tube, wherebythe precursor glass tube is formed.
 43. A process in accordance withclaim 42 comprising the following step (III) after step (II): (III)removing the surface part of the glass plate resulting from the outsidetube.
 44. A process in accordance with claim 35, wherein step (Ia)comprises the following steps: (I0) providing a precursor glass cylinderhaving a precursor cylinder axis, a length L₀ in the direction of theprecursor cylinder axis and a precursor cylinder outer diameter OD₀;(I1) thermally reflowing, with optional pressing, the precursor glasscylinder; and (I2) optionally drilling in a direction essentiallyparallel to the precursor cylinder axis to form a cylindrical centercavity, whereby the precursor glass tube is formed to have alongitudinal tube axis, an outer diameter OD₁ and a length L₁ in thedirection of the tube axis, where the tube axis is essentially parallelto the precursor cylinder axis of the precursor glass cylinder, L₁<L₀,and OD₁>OD₀.
 45. A process in accordance with claim 44, wherein0.3L₀≦L₁≦0.8L₀.
 46. A process in accordance with claim 44, wherein: instep (I0), the precursor glass cylinder comprises an inner glass cane;said inner glass cane is located approximately at the center of theprecursor glass cylinder and has a diameter of ID₀; and in step (I2),the inner glass cane is essentially completely removed.
 47. A process inaccordance with claim 44, wherein in step (I0), the precursor glasscylinder comprises a mandrel in essentially the central portion.
 48. Aprocess in accordance with clam 47, wherein the mandrel is maintained inplace during step (I1), and removed after step (I1).
 49. A process inaccordance with claim 48, wherein the dimension of the mandrel isessentially not changed during step (I1).
 50. A process in accordancewith claim 47, wherein the mandrel is inserted into a glass tube.
 51. Aprocess in accordance with claim 44, wherein in step (I0), the precursorglass cylinder comprises an outside tube having composition and/orproperties differing from those of the glass enclosed in the outsidetube.
 52. A process in accordance with claim 51 comprising the followingstep (III) after step (II): (III) removing the surface part of the glassplate resulting from the outside tube.
 53. A process in accordance withclaim 46, wherein after step (I2), the precursor glass tube has an innercylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁.
 54. Aprocess in accordance with claim 44, wherein in step (I0), the providedprecursor glass cylinder has an inner cylindrical cavity the axis ofwhich is parallel to the precursor cylinder axis, and the innercylindrical cavity has a diameter of ID₀.
 55. A process in accordancewith claim 54, wherein after step (I2), the ready-to-flow glass tube hasan inner cylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁.56. A process in accordance with claim 11, wherein in step (II), thereflow is done without external mechanical assistance.
 57. A process inaccordance with claim 14, wherein after step (II), the striae arere-oriented to be essentially parallel to the two major surfaces of theresultant glass plate.
 58. A process in accordance with claim 14,wherein after step (II), when viewed in the direction of the opticalaxis of the resultant glass plate, the glass plate is essentially freeof striae.
 59. A process in accordance with claim 58, wherein after step(II), when viewed in at least one direction perpendicular to the opticalaxis of the resultant glass plate, the glass plate is essentially freeof striae.
 60. A process in accordance with claim 58, wherein after step(II), when viewed in the direction of the center tube axis of theready-to-flow glass tube, the resultant glass plate is essentially freeof striae.
 61. A process in accordance with claim 11, wherein in step(II), external forces other than gravity of the ready-to-flow notchedglass tube are exerted on the ready-to-flow notched glass tube tofacilitate the reflow of the glass.
 62. A process in accordance withclaim 61, wherein in step (II), the external forces other than gravityof the ready-to-flow notched glass tube are exerted on the two sidesurfaces of the notch.
 63. A process in accordance with claim 61,wherein in step (II), the external forces are applied by a plunger tothe surfaces of the inner cavity and/or the side surfaces of the notch.64. A process in accordance with claim 63, wherein in step (II), theexternal forces are applied via an articulating mandrel and a plunger.65. A process in accordance with claim 11, wherein step (II) comprisesthe following steps: (IIa) placing the ready-to-flow notched glass-tubeon an essentially horizontal longitudinal mandrel, with the mandrelinserting into the inner cavity of the tube, and the notch placed facingsideways; (IIb) allowing the lower part of the notched glass tube toroll out to an essentially vertical position while restricting the upperpart from rolling out, to result in a partially rolled out glass piece;(IIc) placing the partially rolled out glass piece on a surface; and(IId) allowing the partially rolled out glass piece to roll-out on thesurface to form an essentially flat glass plate.
 66. A process inaccordance with claim 65, wherein in step (IId), an external force isimposed on the partially rolled-out glass piece to mechanically assistthe roll out of the glass piece.
 67. A process in accordance with claim66, wherein in step (IId), the external force is imposed via a mandrel.68. A process in accordance with claim 11, wherein in step (II), thetemperature elevation rate is between 50-600° C./minute between theannealing point of the glass and the highest temperature.
 69. A processin accordance with claim 68, wherein in step (II), the temperatureelevation rate is between 180-600° C./minute between the annealing pointof the glass and the highest temperature.
 70. A process in accordancewith claim 11, wherein in step (II), the temperature is held for aperiod of between 10 minutes to 5 hours, at a temperature between theannealing point and the devitrification range of the glass.
 71. Aprocess for reforming a glass cylinder comprising the following steps:(I.0) providing a precursor glass cylinder having a precursor cylinderaxis, a length L₀ in the direction of the precursor cylinder axis and aprecursor cylinder outer diameter OD₀; (I.1) thermally reflowing, withoptional pressing, the precursor glass cylinder; and (I.2) optionallydrilling in a direction essentially parallel to the precursor cylinderaxis to form a cylindrical center cavity, whereby a reformed glasscylinder is formed to have a longitudinal reformed cylinder axis, anouter diameter OD₁ and a length L₁ in the direction of the reformedcylinder axis, where the reformed cylinder axis is essentially parallelto the precursor glass cylinder axis of the precursor glass cylinder,L₁<L₀, and OD₁>OD₀.
 72. A process in accordance with claim 71, wherein0.3L₀≦L₁≦0.8L₀.
 73. A process in accordance with claim 71, wherein: instep (I.0), the precursor glass cylinder comprises an inner glass cane;said inner glass cane is located approximately at the center of theprecursor glass cylinder and has a diameter of ID₀; and in step (I.2),the inner glass cane is essentially completely removed.
 74. A process inaccordance with claim 71, wherein in step (I.0), the precursor glasscylinder comprises a mandrel in essentially the central portion.
 75. Aprocess in accordance with clam 74, wherein the mandrel is maintained inplace during step (I.2), and removed after step (I.2).
 76. A process inaccordance with claim 75, wherein the dimension of the mandrel isessentially not changed during step (I.2).
 77. A process in accordancewith claim 74, wherein the mandrel is inserted into a glass tube.
 78. Aprocess in accordance with claim 71, wherein in step (I.0), theprecursor glass cylinder comprises an outside tube having the same ordiffering composition and/or properties.
 79. A process in accordancewith claim 78 comprising the following step (III) after step (II): (III)removing the surface part of the glass plate resulting from the outsidetube.
 80. A process in accordance with claim 73, wherein after step(I.2), the precursor glass tube has an inner cylindrical cavity with adiameter ID₁, and OD₀−ID₀<OD₁−ID₁.
 81. A process in accordance withclaim 71, wherein in step (I.0), the provided precursor glass cylinderhas an inner cylindrical cavity the axis of which is essentiallyparallel to the precursor glass cylinder axis, and the inner cylindricalcavity has a diameter of ID₀.
 82. A process in accordance with claim 81,wherein after step (I.2), the ready-to-flow glass tube has an innercylindrical cavity with a diameter ID₁, and OD₀−ID₀<OD₁−ID₁.